Defenses start at level 0 and can be upgraded up to the max of level 5 at varying mana costs. Along with the normal damage, etc., increase they get a hidden bonus to the stat modifier. Level 0 gets none, level 1 gets 2, level 2 gets 5, level 3 gets 12, level 4 gets 22 and level 5 gets 36. There is a 33% bonus to damage when the hero that summoned the defense is active.